Flu, also known as influenza, is a communicable disease caused by the influenza virus. It spreads easily from person to person through respiratory droplets when an infected person talks, coughs, or sneezes. This means that if someone is infected with the flu, they can easily transmit the virus to others nearby. In contrast, diabetes mellitus, sickle cell anemia, and cancer are not communicable diseases; they cannot be transmitted from one person to another. Diabetes is a metabolic disorder, sickle cell anemia is a genetic condition, and cancer is a group of diseases characterized by uncontrolled cell growth.